Power supply efficiency is without a doubt the most talked about quality in recent years. It's the percentage of current that actually reaches the PC components. If components use a total of 400W, a system with an 80 percent efficient PSU uses 400 / 0.80 = 500W. The same PC with a 90 percent efficient PSU only draws 400 / 0.90 = 444W. It can potentially make a difference in your energy bill, but also translates to other areas. The wasted capacity is turned into heat. The more efficient the PSU, the cooler it stays, the less cooling you need, which improves the lifespan of the components.
In budget power supplies its common that the manufacturers cut corners, and lower quality parts can have a negative influence on the efficiency. So there's definitely a correlation between efficiency and the overall quality of a PSU. The most efficient power supply in our 300W test drew about 326W of power. The least efficient PSU used about 419 watts of power, so that can indeed make a difference on your electricity bill.
The most efficient power supply overall in our test is the Seasonic Platinum Series Fanless 400W with an average of 91.23%. The fact that it doesn't have a fan that uses power helps here. Two other PSU ended up above 90.5%: the Antec EarthWatts Platinum 450W and the Be Quiet! Straight Power E9 450W. With prices around 85 euros these two PSUs are pretty expensive compared to most other PSUs in the test, but compared to the Seasonic it's ok.
The FSP FSP350-60EGN, Huntkey Jumper 300G and Seasonic G-Series 360W – all with 80Plus Gold certification – are also very efficient, with values above 89%. The other PSUs in the test are around 86.5%. Three power supplies are below 80%, far even. The Huntkey CP 400W (76.0%), Huntkey CP 350W (72.7%) and Cooler Master Elite 400W (72.7%) have such low efficiency scores that it's a reason not to buy them despite their affordable price.